This invention relates generally to a coal gasification process for producing synthesis gas and, more particularly, to an improvement in the process for treating the raw producer gas from the gasifier to effect the removal of pollutants therefrom and the subsequent elimination of these pollutants by reintroducing them into the gasifier.
The production of synthetic or fuel gas from the solid carbonaceous fuels, especially coal in the form of anthracite, bituminous, lignite or peat, has been utilized for a considerable period of time and has recently undergone significant improvements due to the increased energy demand. Fuel gas may be produced by heating coal with reactive gases, such as air or oxygen, in the presence of steam in a gasification zone to obtain the fuel gas which is withdrawn from the gasification zone and subjected to several cleansing operations to rid it of various contaminants which are formed or liberated from the coal during the gasification operation. These contaminants can readily become environmental pollutants if not properly treated during the gasification operation. For example, materials often found in the fuel gas or producer gas include tars, light oils, hydrogen sulfide, ammonia, cyanides, phenols, various halogens and particulates in the form of carbon, ash, and coal, as well as trace metals. The extent of the pollutants in the coal is determined by the type of coal and the particular gasification process utilized as well as the operating conditions. In any event, the disposal and control of these pollutants have become major problems in the gasification processes which must be satisfactorily handled in order to make coal gasification a viable process without suffering attendant pollution problems.
As the producer gas is discharged from the gasifier, it is usually subjected to a cooling and cleaning operation involving a scrubbing technique wherein the gas is introduced into a scrubber and contacted with a water spray which cools the gas and condenses such condensables as tar, oil and organics. The initially cooled gas may then be treated to desulfurize the gas prior to utilization of the producer gas. The water used for the scrubbing operation becomes what is commonly known as "dirty water," since it is contaminated with tars, various organic materials, and soluble gases. This dirty water may be subjected to a variety of steps which may include the decantation of the heavy and light portions, the stripping of the water to remove such gases as hydrogen sulfide, ammonia, and also solvent extraction steps to remove the phenols, cyanides and the other inorganic anions, such as the halogens. This water after such treatment still contains a considerable amount of materials, especially organics, which must be disposed of or somehow handled without creating an environmental pollution problem. The common practice is to use a containment pond or other disposal or containment areas for the dirty water. However, such disposal techniques have many pitfalls due to the high chemical oxygen demand of the water and the possibility of the material entering the water table or presenting other ecological polluting problems such as invading surface waters.